1. Field of the Invention
This invention relates to a fine structure evaluation apparatus and a fine structure evaluation method. The present invention is suitable for, for example, edge evaluation apparatuses, positioning apparatuses, pattern line width measuring apparatuses, foreign particle inspection apparatuses and the like, which perform optical evaluation of a fine structure, detection of foreign particles, and the like by irradiating light from a light source means onto a minute region on an object to be measured, photoelectrically detecting light scattered by a fine structure of the object or a foreign particle on the object by a detecting system, and analyzing and evaluating signals from the detecting system.
2. Description of the Related Art
Various kinds of fine structure evaluation apparatuses for detecting and evaluating an outer shape, such as an edge, a line width or the like, of a fine pattern, have been proposed. For example, there have been known edge detection apparatuses for detecting an edge portion of a pattern on the surface of a wafer as a fine structure, and positioning a reticle with the wafer.
FIG. 1 is a schematic diagram of a principal part of an optical system of a known edge detection apparatus. In this apparatus, the beam size of laser light from a laser light source 2a is expanded by a beam expander 3a, and is adjusted by a slit 4a, from which the light beam emanates. The light beam is focused by a focusing lens 6a to illuminate an edge region 10a of a pattern 10 on a wafer surface 8a. In FIG. 1, curves A and B on the x and y coordinate axes schematically represent the intensity distributions of the light beam.
A greater amount of scattered light is generated from the edge portion 10a of the pattern 10 than from other portions. Accordingly, the presence or the intensity of the scattered light is detected by photoelectric detectors 14, and the position of the edge portion 10a of the pattern 10 is detected utilizing signals from the photoelectric detectors 14. That is, positional information with respect to the pattern 10 is detected.
The edge detection apparatus shown in FIG. 1 scans the object 10 while focusing the laser light into a minute spot, and photoelectrically detects the presence or the intensity distribution of the light scattered by the edge portion 10a by the photoelectric detectors 14 disposed at an angle with respect to the optical axis of the focused light beam, whereby the position of the edge portion 10a is detected. At that time, light beam deforming means 4a comprising the slit for deforming the shape of the projected spot of the laser light into the shape of an ellipse is provided, whereby the width of the spot in the direction of the minor axis of the ellipse is reduced to increase resolution in detection.
FIG. 2 is a schematic diagram of a principal part of a pattern line size measuring apparatus proposed in Japanese Patent Application Public Disclosure (Kokai) No. 62-118206 (1987).
In FIG. 2, a laser light beam 160 from a laser light source 156 passing through a half-mirror 157 is focused by an objective lens 155 to illuminate a pattern 153 on the surface of an object 152 to be inspected mounted on a vibrating stage 151. Light scattered by the pattern 153 and a reflected light beam 161 are focused by the objective lens 155, and the pattern 153 is imaged onto the surface of a photosensor 158. At that time, the vibrating stage 151 is vibrated in the same direction as the measuring direction of the pattern 153 by a vibrator 154. A signal 162 obtained from the photosensor 158 is processed by a signal processor 159 to obtain the size of the pattern 153.
As apparatuses for detecting foreign particles on an object to be inspected, foreign particle inspection apparatuses have been proposed in which laser light is obliquely projected onto an object, and light scattered by a foreign particle is detected by photosensors disposed at certain positions.
In the edge detection apparatus shown in FIG. 1, since light scattered by the fine structure is detected by the photoelectric detectors 14 fixedly disposed at a predetermined angle with respect to the focused light, the following problems are present:
(1-1) Since the presence of scattered light, or a change in the intensity distribution of only a part of scattered light is detected, only an approximate position of the border of the edge portion can be detected.
(1-2) Since analysis processing of fine structure information included in scattering distribution characteristics of scattered light in a manner to be described later is not performed, the shape of the fine structure cannot be evaluated.
(1-3) It is difficult to optically evaluate the fine structure of the object with a high resolution.
In the conventional pattern line size measuring apparatus shown in FIG. 2, the photosensor 158 is disposed at a position optically conjugate to the pattern 153 on the object 152, and a signal representing light reflected and scattered by the pattern 153 is detected by the photosensor 158, whereby the size of the pattern 153 is measured. Accordingly, in the conventional pattern line size measuring apparatus, the spot size of the light beam obtained by the illuminating optical system is on the order of about one wavelength of the illuminating light beam. Hence, if the line width of the pattern is smaller than the spot size, it is impossible to measure the line width and the height of the pattern line from a reference surface.
Furthermore, in the conventional foreign particle inspection apparatus, it is difficult to discriminate between light scattered by a fine structure of an object and light scattered by a foreign particle, and therefore to optically inspect a fine foreign particle with a high resolution.